Neutral particle beam processing apparatus

ABSTRACT

A neutral particle beam processing apparatus comprises a workpiece holder ( 20 ) for holding a workpiece (X), a plasma generator for generating a plasma in a vacuum chamber ( 3 ) by applying a high-frequency electric field, an orifice electrode ( 4 ) disposed between the workpiece holder ( 20 ) and the plasma generator, and a grid electrode ( 5 ) disposed upstream of the orifice electrode ( 4 ) in the vacuum chamber ( 3 ). The orifice electrode ( 4 ) has orifices ( 4   a ) defined therein. The neutral particle beam processing apparatus further comprises a voltage applying unit for applying a voltage between the orifice electrode ( 4 ) which serves as an anode and the grid electrode ( 5 ) which serves as a cathode, while the high-frequency electric field applied by the plasma generator is being interrupted, to accelerate negative ions in the plasma generated by the plasma generator and pass the accelerated negative ions through the orifices ( 4   a ) in the orifice electrode ( 4 ).

TECHNICAL FIELD

[0001] The present invention relates to a neutral particle beamprocessing apparatus, and more particularly to a neutral particle beamprocessing apparatus for generating a highly directional and highlydense neutral particle beam from a high-density plasma and processing aworkpiece with the generated neutral particle beam.

BACKGROUND ART

[0002] In recent years, semiconductor integrated circuits, informationstorage media such as hard disks, micromachines, and the like have beenprocessed in highly fine patterns. In the fields of processing suchworkpieces, attention has been attracted to the use of an energetic beamsuch as a high-density ion beam which is highly linear, i.e., highlydirectional, and has a relatively large beam diameter. For example, theenergetic beam is applied to a workpiece for depositing a film thereonor etching the workpiece.

[0003] As beam sources of such energetic beams, there have been usedbeam generators which generate various kinds of beams including apositive ion beam, a negative ion beam, and a radical beam. The positiveion beam, the negative ion beam, or the radical beam is applied to adesired area of a workpiece from the beam source, for thereby locallydepositing a film on the workpiece, etching the workpiece, modifying thesurface of the workpiece, or joining or bonding parts of the workpiece.

[0004] In the case of a beam source which applies charged particles suchas positive ions or negative ions to a workpiece, an insulated workpiececannot be processed because of a charge build-up phenomenon in whichelectric charges are built up on the workpiece. Further, since the ionbeam emitted from the beam source tends to spread due to thespace-charge effect, the workpiece cannot be processed in a finepattern.

[0005] In order to solve the above problems, there has been proposed amethod of introducing electrons into the ion beam to neutralize theelectric charges. This method can balance the electric charges on theworkpiece on the whole. However, since local unbalance of the electriccharges still remains on the workpiece, the workpiece cannot beprocessed in a fine pattern.

[0006] In the case where ions are extracted from a plasma source andapplied to a workpiece, if a radiation (e.g., an ultraviolet ray)produced by the plasma source is applied to the workpiece, then theradiation adversely affects the workpiece. Thus, it is necessary toshield the workpiece from an adverse radiation (e.g., an ultravioletray) emitted from the plasma source.

DISCLOSURE OF INVENTION

[0007] The present invention has been made in view of the abovedrawbacks. It is therefore an object of the present invention to providea neutral particle beam processing apparatus which can apply anenergetic beam having a large beam diameter to a workpiece with aninexpensive and compact structure, and can neutralize ions with a highneutralization efficiency to process the workpiece without a chargebuild-up or damage.

[0008] According to a first aspect of the present invention, there isprovided a neutral particle beam processing apparatus comprising: aworkpiece holder for holding a workpiece; a plasma generator forgenerating a plasma in a vacuum chamber by applying a high-frequencyelectric field; an orifice electrode disposed between the workpieceholder and the plasma generator, the orifice electrode having orificesdefined therein; a grid electrode disposed upstream of the orificeelectrode in the vacuum chamber; and a voltage applying unit forapplying a voltage between the orifice electrode which serves as ananode and the grid electrode which serves as a cathode, while thehigh-frequency electric field applied by the plasma generator is beinginterrupted, to accelerate negative ions in the plasma generated by theplasma generator and pass the accelerated negative ions through theorifices in the orifice electrode.

[0009] With the above arrangement, since the workpiece can be processedby a neutral particle beam having no electric charges but having a largetranslational energy, various processes including an etching process anda deposition process can be performed on the workpiece with highaccuracy in such a state that an amount of charge build-up is reduced.Particularly, when the orifice electrode is used for neutralizing thenegative ions, a high neutralization efficiency can be obtained, andhence a beam diameter of an energetic beam can be increasedinexpensively without increasing the size of the apparatus. Further,since the generated plasma is isolated from the workpiece by the orificeelectrode, a radiation produced by the plasma is not substantiallyapplied to the workpiece. Therefore, it is possible to reduce adverseeffects on the workpiece due to the radiation such as an ultraviolet raywhich would otherwise damage the workpiece.

[0010] According to a second aspect of the present invention, there isprovided a neutral particle beam processing apparatus comprising: aworkpiece holder for holding a workpiece; an orifice electrode disposedin a vacuum chamber, the orifice electrode having orifices definedtherein; a second electrode disposed upstream of the orifice electrodein the vacuum chamber; a first voltage applying unit for applying ahigh-frequency voltage between the orifice electrode and the secondelectrode to generate a plasma between the orifice electrode and thesecond electrode; and a second voltage applying unit for applying avoltage between the orifice electrode which serves as an anode and thesecond electrode which serves as a cathode, while the high-frequencyelectric field applied by the first voltage applying unit is beinginterrupted, to accelerate negative ions in the plasma generated by thefirst voltage applying unit and pass the accelerated negative ionsthrough the orifices in the orifice electrode.

[0011] With the above arrangement, the orifice electrode serves not onlyto neutralize the negative ions, but also to generate the plasma.Therefore, a high neutralization efficiency can be obtained by theorifice electrode, and simultaneously it is not necessary to provide aseparate plasma generator for generating a plasma. Thus, the neutralparticle beam processing apparatus can be made compact in structure, anda beam diameter of an energetic beam can be increased inexpensively.

[0012] Preferably, the orifice electrode has a thickness which is atleast twice the diameter of the orifices defined therein. When theorifice electrode has a thickness which is at least twice the diameterof the orifices, it is possible to increase the probability that thenegative ions are neutralized in the orifices, and to remarkably reducethe intensity of a radiation to be applied to the workpiece from theplasma.

[0013] Preferably, the orifice electrode is made of an electricallyconductive material. With the orifice electrode made of an electricallyconductive material, a positive DC voltage and a negative DC voltage mayselectively be applied to the orifice electrode to accelerate bothpositive ions and negative ions in the plasma. If a low-frequencyvoltage having a frequency of about 400 kHz is applied to the orificeelectrode, then it is possible to accelerate positive ions and negativeions alternately. In this case, the surfaces of the orifice electrodemay be covered with dielectric films.

[0014] The above and other objects, features, and advantages of thepresent invention will be apparent from the following description whentaken in conjunction with the accompanying drawings which illustratespreferred embodiments of the present invention by way of example.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a schematic view showing a whole arrangement of aneutral particle beam processing apparatus according to a firstembodiment of the present invention;

[0016]FIG. 2A is a perspective view showing an orifice electrode and agrid electrode in the neutral particle beam processing apparatus shownin FIG. 1;

[0017]FIG. 2B is a vertical cross-sectional view partially showing theorifice electrode and the grid electrode shown in FIG. 2A;

[0018]FIG. 3 is a timing chart showing operating states of the neutralparticle beam processing apparatus shown in FIG. 1;

[0019]FIG. 4 is a schematic view showing a whole arrangement of aneutral particle beam processing apparatus according to a modificationof the first embodiment of the present invention;

[0020]FIG. 5 is a schematic view showing a whole arrangement of aneutral particle beam processing apparatus according to a secondembodiment of the present invention; and

[0021]FIG. 6 is a timing chart showing operating states of the neutralparticle beam processing apparatus shown in FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] A neutral particle beam processing apparatus according to a firstembodiment of the present invention will be described in detail belowwith reference to FIGS. 1 through 3.

[0023]FIG. 1 is a schematic view showing a whole arrangement of aneutral particle beam processing apparatus according to a firstembodiment of the present invention, with electric components in blockform. As shown in FIG. 1, the neutral particle beam processing apparatuscomprises a cylindrical vacuum chamber 3 constituted by a beamgenerating chamber 1 for generating a neutral particle beam and aprocess chamber 2 for processing a workpiece X such as a semiconductorsubstrate, a glass workpiece, an organic workpiece, a ceramic workpiece,or the like. The beam generating chamber 1 of the vacuum chamber 3 haswalls made of quartz glass or ceramics, and the process chamber 2 of thevacuum chamber 3 has walls made of metal.

[0024] The beam generating chamber 1 has a coil 10 disposed therearoundfor inductively coupled plasma (ICP). The coil 10 is housed in awater-cooled tube having an outside diameter of 8 mm, for example. Thecoil 10 of about two turns is wound around the beam generating chamber1. The coil 10 is electrically connected through a matching box 100 to ahigh-frequency power supply 101, which applies a high-frequency voltagehaving a frequency of about 13.56 MHz, for example, to the coil 10. Whena high-frequency current is supplied from the high-frequency powersupply 101 via the matching box 100 to the coil 10, an induced magneticfield is produced in the beam generating chamber 1 by the coil 10. Thevarying magnetic field induces an electric field, which accelerateselectrons to generate a plasma in the beam generating chamber 1. Thus,the coil 10, the matching box 100, and the high-frequency power supply101 constitute a plasma generator for generating a plasma in the beamgenerating chamber 1.

[0025] The beam generating chamber 1 has a gas inlet port 11 defined inan upper portion thereof for introducing a gas into the beam generatingchamber 1. The gas inlet port 11 is connected through a gas supply pipe12 to a gas supply source 13, which supplies a gas such as SF₆, CHF₃,CF₄, Cl₂, Ar, O₂, N₂, and C₄F₈ to the beam generating chamber 1.

[0026] The process chamber 2 houses a workpiece holder 20 therein forholding a workpiece X. The workpiece X is placed on an upper surface ofthe workpiece holder 20. The process chamber 2 has a gas outlet port 21defined in a sidewall thereof for discharging the gas from the processchamber 2. The gas outlet port 21 is connected through a gas outlet pipe22 to a vacuum pump 23, which operates to maintain the process chamber 2at a predetermined pressure.

[0027] An orifice plate (orifice electrode) 4 made of an electricallyconductive material such as graphite is disposed in the lower end of thebeam generating chamber 1 and electrically grounded. A thin-plate gridelectrode 5 made of an electrically conductive material is disposedabove the orifice electrode 4. The grid electrode 5 is electricallyconnected to a bipolar power supply (voltage applying unit) 102.

[0028]FIG. 2A is a perspective view showing the orifice electrode 4 andthe grid electrode 5, and FIG. 2B is a vertical cross-sectional viewpartially showing the orifice electrode 4 and the grid electrode 5 shownin FIG. 2A. As shown in FIGS. 2A and 2B, the orifice electrode 4 has anumber of orifices 4 a defined therein, and the grid electrode 5 has anumber of grid holes 5 a defined therein. The grid electrode 5 maycomprise a meshed wire, a punching metal, or the like.

[0029] The high-frequency power supply 101 which is connected to thecoil 10 is connected a modulator 103, and the bipolar power supply 102which is connected to the grid electrode 5 is connected to a modulator104. Thus, the high-frequency power supply 101 and the bipolar powersupply 102 are connected to each other through the modulators 103, 104.The application of the voltage by the bipolar power supply 102 issynchronized with the application of the voltage by the high-frequencypower supply 101, based on synchronizing signals transmitted between themodulators 103, 104.

[0030] Operation of the neutral particle beam processing apparatusaccording to the first embodiment will be described below. FIG. 3 is atiming chart showing operating states of the neutral particle beamprocessing apparatus shown in FIG. 1. In FIG. 3, Va represents thepotential of the coil 10, Te the electron temperature in the beamgenerating chamber 1, ne the electron density in the beam generatingchamber 1, ni⁻ the negative ion density in the beam generating chamber1, and Vb the potential of the grid electrode 5. The timing chart isschematically shown in FIG. 3, and the shown frequencies are differentfrom the actual frequencies, for example.

[0031] The vacuum pump 23 is driven to evacuate the vacuum chamber 3,and then a gas such as SF₆, CHF₃, CF₄, Cl₂, Ar, O₂, N₂, or C₄F₈ isintroduced from the gas supply source 13 into the beam generatingchamber 1. As shown in FIG. 3, a high-frequency voltage having afrequency of about 13.56 MHz is applied to the coil 10 for 10microseconds by the high-frequency power supply 101, so that ahigh-frequency electric field is produced in the beam generating chamber1. The gas introduced into the beam generating chamber 1 is ionized byelectrons that are accelerated by the high-frequency electric field, forthereby generating a high-density plasma in the beam generating chamber1. The plasma is mainly composed of positive ions and heated electrons.

[0032] Then, the high-frequency voltage applied by the high-frequencypower supply 101 is interrupted for 100 microseconds. Thereafter, thehigh-frequency voltage is applied again to the coil 10 for 10microseconds by the high-frequency power supply 101 to heat theelectrons in the plasma in the beam generating chamber 1. Thus, theabove cycle is repeated. In this manner, the application of thehigh-frequency voltage for 10 microseconds and the interruption of thehigh-frequency voltage for 100 microseconds are alternately repeated.The period of time (100 microseconds) for which the high-frequencyvoltage is interrupted is sufficiently longer than a period of time inwhich the electrons in the plasma are attached to the residual processgas to generate negative ions, and sufficiently shorter than a period oftime in which the electron density in the plasma is lowered toextinguish the plasma. The period of time (10 microseconds) for whichthe high-frequency voltage is applied is long enough to recover theenergy of the electrons in the plasma which has been lowered during theinterruption of the high-frequency voltage.

[0033] Negative ions can be generated efficiently and continuously byinterrupting the high-frequency voltage after the energy of theelectrons is increased in the plasma. While ordinary plasmas are mostlycomposed of positive ions and electrons, the neutral particle beamprocessing apparatus according to the present embodiment can efficientlygenerate a plasma in which positive ions and negative ions coexisttherein. Although the high-frequency voltage is interrupted for 100microseconds in the above example, it may be interrupted for a period oftime ranging from 50 to 100 microseconds to generate a large quantity ofnegative ions as well as positive ions in the plasma.

[0034] After 50 microseconds from the time when the high-frequencyvoltage applied by the high-frequency power supply 101 is stopped, a DCpulsed voltage of −100 V is applied to the grid electrode 5 for 50microseconds by the bipolar power supply 102. The application of the DCvoltage lowers the potential Vb of the grid electrode 5 below thepotential (ground potential) of the orifice electrode 4. Thus, apotential difference is produced between the orifice electrode 4 and thegrid electrode 5. In this state, the orifice electrode 4 serves as ananode, and the grid electrode 5 serves as a cathode. Therefore, thenegative ions 6 (see FIG. 2B) that have passed through the gridelectrode 5 toward the orifice electrode 4 are accelerated toward theorifice electrode 4 by the potential difference and introduced into theorifices 4 a defined in the orifice electrode 4.

[0035] Most of the negative ions 6 that are passing through the orifices4 a in the orifice electrode 4 are collided with the sidewall surfacesof the orifices 4 a and hence neutralized in the vicinity of solidsidewall surfaces of the orifices 4 a, or are collided with gasmolecules remaining within the orifices 4 a and hence neutralized bycharge exchange with the gas molecules. Thus, the negative ions 6 areconverted into neutral particles 7 (see FIG. 2B). The negative ions 6that have been neutralized when passing through the orifices 4 a, i.e.,the neutral particles 7, are then emitted as an energetic beam into theprocess chamber 2. The neutral particles 7 travel directly in theprocess chamber 2 and are applied to the workpiece X placed on theworkpiece holder 20, for thereby etching the surface of the workpiece X,cleaning the surface of the workpiece X, modifying (e.g., nitriding oroxidizing) the surface of the workpiece X, or depositing a film on theworkpiece X.

[0036] The orifice electrode 4 serves not only to neutralize thenegative ions, but also to prevent a radiation produced by the plasmafrom being applied to the workpiece X. Specifically, since the beamgenerating chamber 1 where the plasma is generated is isolated from theworkpiece X by the orifice electrode 4, the radiation produced by theplasma is not substantially applied to the workpiece X. Therefore, it ispossible to reduce adverse effects on the workpiece X due to theradiation such as an ultraviolet ray which would otherwise damage theworkpiece X.

[0037] The orifice electrode 4 should preferably have a thickness “l”(see FIG. 2B) which is at least twice the diameter “d” (see FIG. 2B) ofthe orifices 4 a. When the orifice electrode 4 has a thickness “l” whichis at least twice the diameter “d” of the orifices 4 a, it is possibleto increase the probability that the negative ions are neutralized inthe orifices 4 a, and to remarkably reduce the intensity of a radiationto be applied to the workpiece X from the plasma.

[0038] Some charged particles may pass through the orifices 4 a in theorifice electrode 4. In order to prevent such charged particles frombeing applied to the workpiece X, a deflector or an electron trap may bedisposed downstream of the orifice electrode 4. A voltage is applied tothe deflector in a direction perpendicular to a beam traveling directionto change the traveling direction of charged particles, for therebypreventing the charged particles from being applied to the workpiece X.The electron trap produces a magnetic field of about 100 gauss in adirection perpendicular to a beam traveling direction to change thetraveling direction of electrons, for thereby preventing the electronsfrom being applied to the workpiece X.

[0039] As well known in the art, when an insulated workpiece such as aworkpiece made of glass or ceramics is processed, charge build-up may bedeveloped on the surface of the insulated workpiece. However, byapplying neutralized particles to the insulating workpiece as describedabove, various processes including an etching process and a depositionprocess can be performed on the insulating workpiece with high accuracyin such a state that an amount of charge build-up is reduced. Varioustypes of gases may be introduced into the beam generating chamber 1according to the type of process to be performed on the workpiece X. Forexample, in a dry etching process, oxygen or a halogen gas mayselectively be used according to the kind of the workpiece X.

[0040] In the present embodiment, it is desirable to introduce a gasthat is liable to generate negative ions, such as O₂, Cl₂, SF₆, CHF₃, orC₄F₈, into the beam generating chamber 1. When the application of thehigh-frequency voltage is interrupted after a high-density plasma isgenerated by the aforementioned high-frequency inductive coupling (ICP)with use of the above gas, a large number of negative ions can begenerated in the plasma. Therefore, it is easy to accelerate andneutralize the negative ions.

[0041] In the first embodiment, the grid electrode 5 is positioneddownstream of the coil 10. However, the grid electrode may be positionedupstream of the coil 10. In such a case, the grid electrode may have nogrid holes therein. FIG. 4 is a schematic view showing a wholearrangement of a neutral particle beam processing apparatus where a gridelectrode 50 is disposed upstream of the coil 10. In the neutralparticle beam processing apparatus shown in FIG. 4, negative ions in aplasma generated in the beam generating chamber 1 are accelerated by avoltage applied between the grid electrode 50 and the orifice electrode4.

[0042] In the above embodiment, the plasma is generated with use of acoil for ICP. However, the plasma may be generated with use of anelectron cyclotron resonance source (ECR source), a coil for heliconwave plasma, a microwave, or the like.

[0043] A neutral particle beam processing apparatus according to asecond embodiment of the present invention will be described below withreference to FIGS. 5 and 6. FIG. 5 is a schematic view showing a wholearrangement of a neutral particle beam processing apparatus according toa second embodiment of the present invention, with electric componentsin block form. In FIG. 5, like parts and components are denoted by thesame reference numerals and characters as those of the first embodimentand will not be described below.

[0044] In the present embodiment, the neutral particle beam processingapparatus comprises a vacuum chamber 30 made of metal, i.e., a metallicchamber. As shown in FIG. 5, a thin-plate grid electrode (secondelectrode) 8 made of an electrically conductive material is disposed inan upstream end of the vacuum chamber 30. The vacuum chamber 30 and thegrid electrode 8 are electrically connected to each other andelectrically grounded.

[0045] An AC power supply (first voltage applying unit) 105 and a DCpower supply (second voltage applying unit) 106, which are connectedparallel to each other, are electrically connected to the orificeelectrode 4. The power supplies 105, 106 are also connected tomodulators 107, 108, respectively. The modulator 107 for the AC powersupply 105 and the modulator 108 for the DC power supply 106 aresynchronized with each other by synchronizing signals. The vacuumchamber 30 and the orifice electrode 4 are electrically insulated fromeach other by an insulating material (not shown). The surfaces of theorifice electrode 4 may be covered with dielectric films.

[0046] Operation of the neutral particle beam processing apparatusaccording to the second embodiment will be described below. FIG. 6 is atiming chart showing operating states of the neutral particle beamprocessing apparatus shown in FIG. 5. In FIG. 6, Vc represents thepotential of the AC power supply 105, Te the electron temperature in thebeam generating chamber 1, ne the electron density in the beamgenerating chamber 1, ni⁻ the negative ion density in the beamgenerating chamber 1, Vd the potential of the DC power supply 106, andVe the potential of the orifice electrode 4. The timing chart isschematically shown in FIG. 6, and the shown frequencies are differentfrom the actual frequencies, for example.

[0047] The vacuum pump 23 is driven to evacuate the vacuum chamber 30,and then a gas is introduced from the gas supply source 13 into the beamgenerating chamber 1. As shown in FIG. 6, a high-frequency voltagehaving a frequency of about 13.56 MHz is applied to the orificeelectrode 4 for 10 microseconds by the AC power supply 105, Bo that ahigh-frequency electric field is produced in the beam generating chamber1. The gas introduced into the beam generating chamber 1 is ionized byelectrons that are accelerated by the high-frequency electric field, forthereby generating a high-density plasma in the beam generating chamber1.

[0048] Then, the high-frequency voltage applied by the AC power supply105 is interrupted for 100 microseconds. Thereafter, the high-frequencyvoltage is applied again to the orifice electrode 4 for 10 microsecondsby the AC power supply 105 to heat the electrons in the plasma in thebeam generating chamber 1. Thus, the above cycle is repeated. In thismanner, the application of the high-frequency voltage for 10microseconds and the interruption of the high-frequency voltage for 100microseconds are alternately repeated.

[0049] Negative ions can be generated efficiently and continuously byinterrupting the high-frequency voltage after the energy of theelectrons is increased in the plasma. While ordinary plasmas are mostlycomposed of positive ions and electrons, the neutral particle beamprocessing apparatus according to the present embodiment can efficientlygenerate a plasma in which positive ions and negative ions coexisttherein.

[0050] After 50 microseconds from the time when the high-frequencyvoltage applied by the AC power supply 105 is stopped, a DC voltage of+100 V is applied to the orifice electrode 4 for 50 microseconds by theDC power supply 106. The application of the DC voltage increases thepotential Ve of the orifice electrode 4 above the potential (groundpotential) of the grid electrode 8. Thus, a potential difference isproduced between the orifice electrode 4 and the grid electrode 8. Inthis state, the orifice electrode 4 serves as an anode, and the gridelectrode 8 serves as a cathode. Therefore, the negative ions presentbetween the grid electrode 8 and the orifice electrode 4 are acceleratedtoward the orifice electrode 4 by the potential difference andintroduced into the orifices 4 a defined in the orifice electrode 4.

[0051] Most of the negative ions that are passing through the orifices 4a are neutralized and converted into neutral particles as in the case ofthe first embodiment. The neutral particles are then emitted as anenergetic beam into the process chamber 2. The neutral particles traveldirectly in the process chamber 2 and are applied to the workpiece Xplaced on the workpiece holder 20.

[0052] According to the second embodiment, as described above, byalternately applying the high-frequency voltage and the low-frequencyvoltage between the orifice electrode 4 and the grid electrode 8, aplasma can be generated in the beam generating chamber, and negativeions can be extracted from the generated plasma. Therefore, it is notnecessary to provide a separate plasma generator for generating aplasma. Thus, the neutral particle beam processing apparatus can be madecompact in structure, and a beam diameter of an energetic beam can beincreased inexpensively.

[0053] The frequency of the high-frequency voltage is not limited to13.56 MHz, but may be in the range from 1 MHz to 20 GHz.

[0054] Although certain preferred embodiments of the present inventionhave been shown and described in detail, it should be understood thatvarious changes and modifications may be made therein without departingfrom the scope of the appended claims.

Industrial Applicability

[0055] The present invention is suitable for use in a neutral particlebeam processing apparatus for generating a highly directional and highlydense neutral particle beam from a high-density plasma and processing aworkpiece with the generated neutral particle beam.

1. A neutral particle beam processing apparatus comprising: a workpieceholder for holding a workpiece; a plasma generator for generating aplasma in a vacuum chamber by applying a high-frequency electric field;an orifice electrode disposed between said workpiece holder and saidplasma generator, said orifice electrode having orifices definedtherein; a grid electrode disposed upstream of said orifice electrode insaid vacuum chamber; and a voltage applying unit for applying a voltagebetween said orifice electrode which serves as an anode and said gridelectrode which serves as a cathode, while the high-frequency electricfield applied by said plasma generator is being interrupted, toaccelerate negative ions in the plasma generated by said plasmagenerator and pass the accelerated negative ions through said orificesin said orifice electrode.
 2. A neutral particle beam processingapparatus according to claim 1, wherein said orifice electrode has athickness which is at least twice the diameter of said orifices definedtherein.
 3. A neutral particle beam processing apparatus according toclaim 1, wherein said orifice electrode is made of an electricallyconductive material.
 4. A neutral particle beam processing apparatuscomprising: a workpiece holder for holding a workpiece; an orificeelectrode disposed in a vacuum chamber, said orifice electrode havingorifices defined therein; a second electrode disposed upstream of saidorifice electrode in said vacuum chamber; a first voltage applying unitfor applying a high-frequency voltage between said orifice electrode andsaid second electrode to generate a plasma between said orificeelectrode and said second electrode; and a second voltage applying unitfor applying a voltage between said orifice electrode which serves as ananode and said second electrode which serves as a cathode, while thehigh-frequency voltage applied by said first voltage applying unit isbeing interrupted, to accelerate negative ions in the plasma generatedby said first voltage applying unit and pass the accelerated negativeions through said orifices in said orifice electrode.
 5. A neutralparticle beam processing apparatus according to claim 4, wherein saidorifice electrode has a thickness which is at least twice the diameterof said orifices defined therein.
 6. A neutral particle beam processingapparatus according to claim 4, wherein said orifice electrode is madeof an electrically conductive material.